1. Field of the Invention
The present invention relates generally to apparatus for stretching expandable polymer films such as PTFE films, and more particularly, to an apparatus and method for stretching and expanding polymer films in multiple directions to more uniformly expand such films.
2. Description of the Relevant Art
Polytetrafluoroethylene (PTFE) is an expandable polymer which finds many uses in medicine and industry. As PTFE is stretched during expansion, it forms a porous microstructure of nodes and fibrils. The direction in which the PTFE is stretched may affect the tensile strength of the resulting tube or film as measured in different directions relative to the direction in which the material was expanded.
It is known to stretch PTFE material along a single expansion axis, or uniaxially. It is also known to stretch PTFE along two different axes. For example, U.S. Pat. No. 4,187,390 to Gore discloses the formation of expanded tubes and films made of PTFE. In one example described in such patent, a PTFE film measuring 4" by 4" was stretched to a 16" by 16" film by stretching the film biaxially. The patent specification states that biaxial stretching was performed on a "pantograph"; in describing the use of such pantograph, the patent specification states that the "4".times.4" film was gripped on each side by 13 actuated clamps, which moved apart uniformly on a scissor mechanism."
In addition, U.S. Pat. No. 4,304,010 to Mano describes a prosthetic vascular graft made of PTFE expanded both longitudinally and radially; Mano describes this process as biaxial stretching of the PTFE tube.
PTFE can be stretched to many times its original dimensions. Prior to expansion, the extruded PTFE film is typically heated to approximately within the range of 225-300 degrees Centigrade. Upon reaching such temperature, the film is expanded by stretching to a desired dimension. Following expansion, the PTFE film is heated to a higher sintering temperature of approximately 375 degrees Centigrade or higher to lock the crystalline structure to its expanded dimensions. During this sintering step, the expanded film must be held to its expanded dimensions, or the film will contract partially back toward its pre-expanded shape.
When PTFE films are used for medical/surgical applications, manufacturers and users of PTFE products are often concerned with both tensile strength and suture tear resistance. Tensile strength is a measure of how hard the PTFE material can be stretched along a particular axis before the material breaks. Suture tear resistance is a measure of how much force can be applied to a suture that has been sewn through the PTFE material before such force tears a hole in the PTFE material. Both tensile strength and suture tear resistance can be sensitive to the axis along which expansion was performed during processing of the PTFE material.
When PTFE materials are used as implantable prosthetic devices, such as vessel repair patches or grafts, the surgeon may place the patch or graft in a variety of different orientations, and there is no guarantee that sutures sewn through such PTFE material will necessarily be oriented in the direction of maximum suture tear resistance. For example, if a PTFE film is used as a vessel repair patch, the physician sometimes cuts the patch from a larger film to provide a patch of customized dimensions, and the manufacturer does not know, and can not predict, which way the physician will orient the film when trimming the final patch. Consequently, it would be desirable to produce PTFE films that are expanded in such a way that their characteristics, such as tensile strength and suture tear resistance, are more uniform and not as dependent upon the particular axis or orientation along which the stress forces are applied.
Accordingly, it is an object of the present invention to provide a simple and inexpensive apparatus to stretch and expand films of PTFE and like materials in a multitude of directions outwardly from the center of the original film to provide a multi-directional expansion of the film.
It is a another object of the present invention to provide such an apparatus which conveniently allows the expanded film to be sintered following expansion.
It is still another object of the present invention to provide a method of uniformly expanding PTFE films in a multitude of directions to provide a multi-directional expansion of the film.
It is a still further object of the present invention to provide PTFE films exhibiting substantially uniform tensile strength and suture tear resistance irrespective of the orientation of the film relative to the stress forces being applied.
Still another object of the present invention is to be able to perform the expansion of such film in a sealed oven of relatively small size to closely control the temperature of the film during the expansion process while conveniently maintaining control over the rate at which the film is expanded.
Yet another object of the present invention is to provide apparatus adapted to separately expand a plurality of PTFE films, which expanded films can then be simultaneously sintered in a sintering oven.
A further object of the present invention is to provide an expansion apparatus which is relatively inexpensive to produce and use, yet which uniformly expands a polymer film.
A still further object of the present invention is to provide such an expansion apparatus wherein the expanded film can be easily and quickly locked in its expanded condition and removed from the expansion apparatus prior to sintering.
These and other objects of the invention will become more apparent to those skilled in the art as the description of the present invention proceeds.